Ascending activating reticular system: characteristics, parts and functions.

A summary about the parts, functions and characteristics of this group of nerve cells.

The reticular formation is a brain system that is involved in many functions, all of them fundamental for our survival and proper functioning.

It is made up of two subsystems, one of them being the ascending activating reticular system, a circuit of several nuclei that is involved in waking involved in wakefulness, attention and sleep-wake changes.

This fascinating set of nuclei is responsible for our attention and, in fact, is one of the areas that are activated right now as we read this article. Let's find out more about this interesting system.

What is the ascending activating reticular system?

The ascending reticular activating system or ARAS, known simply as the ascending reticular activating system (ARAS), is one of the two main subunits of the reticular formation. one of the two main subunits of the reticular formation..

The reticular formation consists of a series of interconnected nuclei found throughout the brainstem. The RASA is found together with the other subsystem, which are the descending pathways leading to the spinal cord, via the reticulospinal tracts.

The ascending activating reticular system is a fundamental part of the vertebrate nervous system, since it is responsible for the regulation of wakefulness and sleep-wake transitions.. This structure is mostly composed of nuclei in the thalamus and an important number of dopaminergic, noradrenergic, serotonergic, histaminergic, cholinergic and glutamatergic brain nuclei.

Structure and parts

The ascending activating reticular system is formed by several circuits that connect the dorsal part of the posterior midbrain (midbrain) and anterior part of the brainstem or pons. These circuits are directed towards the cerebral cortex by means of different pathways that project pathways that project through the thalamus and hypothalamus..

As a whole, the RAAS is composed of more than 20 nuclei on each side of the superior brainstem, pons, medulla and posterior hypothalamus. These neurons release different transmitters, among which we can find dopamine, norepinephrine, serotonin, histamine, acetylcholine and glutamate.. These neurons exert influence at the cortical level thanks to direct and indirect axonal projections through links at the thalamic level.

The thalamic pathway consists mostly of cholinergic neurons in the pontine tegmentum, while the hypothalamic pathway is composed mostly of monoamine-releasing neurons. These are dopamine, norepinephrine, serotonin and histamine.

As already mentioned, there are also neurons in the ascending reticular activating system that release glutamate. These neurons were identified relatively recently, and have been related to the monoaminergic and cholinergic nuclei. The glutamatergic component of the RAAS includes one nucleus in the hypothalamus and several in the brainstem.

We will now go into more detail about which parts make up each of the nuclei specialized in a different type of neurotransmitter.

1. Dopaminergic nuclei

The dopaminergic nuclei are located in the ventral tegmental area and in the pars compacta of the substantia nigra. of the substantia nigra.

2. Noradrenergic nuclei

Among the noradrenergic nuclei we have the Locus coeruleus and noradrenergic nuclei of the brainstem.

3. Serotonergic nuclei

The serotonergic nuclei are the dorsal and medial raphe nucleus.

4. Histaminergic nucleus

The histaminergic nucleus is the tuberomammillary nucleus.

5. Cholinergic nuclei

Among the cholinergic nuclei we have the cholinergic nuclei of the prosencephalon and the nuclei of the pontine tegmentum, namely the laterodorsal and pedunculopontine nuclei..

6. Glutamatergic nuclei

At brainstem level we have the parabrachial nucleus, the precoeruleus and the pedunculopontine tegmental nucleus. At the thalamic level, we have the supramammillary nucleus.

7. Thalamic nuclei

In the thalamus we have the thalamic reticular nucleus and the intralaminar nucleus, which includes the centromedian.

Functions

The ascending activating reticular system is a very important network of nuclei, given that it their functions are of vital importance for the organism to remain active and conscious of its functioning. and is aware of its functioning.

Consciousness

Consciousness, understood in the sense of being aware of one's own state, of one's existence, is a human and other animals' capacityis a human capacity and that of other animals due to the functioning of the RAAS.

2. Regulation of sleep-wake transitions

The ascending activating reticular system is responsible for the physiological changes of a state of sleep-wakefulness. physiological changes from a state of deep sleep to a state of wakefulness, transitions that are reversible and reversible.These transitions are reversible and necessary for the organism.

The ventrolateral preoptic nucleus of the hypothalamus inhibits the neural circuits responsible for the waking state. When this same nucleus is activated it causes the sleep state to occur.

During the sleep state the neurons of the RASA have a lower firing rate, whereas it is higher when the neurons of the hypothalamus are in the sleep state.while it is higher during wakefulness. In order to enter deep sleep it is necessary that there is a reduction of the ascending afferent activity that reaches the cortex, something that occurs by suppression of the ascending reticular system.

3. Attention

The ascending activating reticular system is also involved in the transitions from a state of relaxed consciousness to a state of deep sleep. from a state of relaxed and distracted consciousness to periods of high attention..

This is due to increased Blood flow in the regions where the nuclei of this subsystem are located, blood flow which indicates that there is an increase in neuronal activity in the reticular formation of the midbrain and the intralaminar nuclei of the thalamus.

Clinical Significance

Knowing the functions we have just seen, it is possible to understand the great clinical importance of the ascending activating reticular system, especially when there are massive lesions in the system. Damage to this set of nuclei can result in states of reduced consciousness, such as coma or brain death, in addition to outright death.such as coma or brain death, as well as absolute death.

In terms of pathology, we can highlight that the RASA is mostly impaired by age.. As we age, this set of nuclei deteriorates and its neuronal activity becomes more dysfunctional. Among the diseases that affect this system we can highlight the following:

Narcolepsy

Narcolepsy can be caused by lesions in the pedunculopontine and laterodorsal tegmental nuclei.. In these nuclei there is a down-regulation of the activation of its neurons, in addition to a loss of orexin peptides, causing the characteristic daytime sleepiness of the disease.

Parkinson's disease

Parkinson's disease is a medical condition that affects the brain, especially dopaminergic production.. However, the cholinergic nuclei are also affected. It should be noted that the RAAS is one of the systems that is most affected when the disease appears.

Bibliographical references:

• Young, G.B.. (2014). Ascending Reticular Activating System (ARAS). 10.1016/B978-0-12-385157-4.00311-0.
• Purves, Dale (2011). Neuroscience (5. ed.). Sunderland, Mass.: Sinauer. pp. 390-395. ISBN 978-0-87893-695-3.
• Augustine JR (2016). "Chapter 9: The Reticular Formation". Human Neuroanatomy (2nd ed.). John Wiley & Sons. pp. 141-153. ISBN 9781119073994. Archived
• Mohan Kumar V, Mallick BN, Chhina GS, Singh B (October 1984). "Influence of ascending reticular activating system on preoptic neuronal activity". Exp. Neurol. 86 (1): 40-52. doi:10.1016/0014-4886(84)90065-7.